The present invention relates, in general, to RF power detectors used in power control circuits and, in particular, to a method and apparatus for preventing power amplifier saturation.
A wireless communication system, for example, a Global System for Mobile (GSM) system is a standardized system established by the European Telecommunication Standards Institute (ETSI) that defines a common method of communication between a mobile telephone (mobile station) and a base station. The GSM system uses a Time Division Multiple Accessing (TDMA) signaling mode to utilize the available channel frequencies. The TDMA signaling mode defines a carrier frequency comprising eight TDMA channels having eight time slots with each time slot allocated to one mobile station within a geographic area. Each TDMA channel has a time duration of 4.615 ms and each timeslot has a duration of 577 xcexcs. Each time slot carries either speech or control data in a burst form.
The wireless communication system may use an Amplitude Modulation (AM) scheme to modulate the carrier frequency with information. The wireless communication system uses a transmitter to transmit the modulated RF signals and a power amplifier to linearly amplify the modulated RF signal to be transmitted. The amplifier is required to ramp up to a specific power level, transmit the signal containing information, and to ramp down to a specified power level in the defined amount of time in order to avoid interference with an adjacent time slot.
Power amplifiers are biased to operate within the specified constraints with maximum efficiency and linearity. The amplifier""s optimized bias point or Q-point should be defined as having a safe zone between the operating point and the 1 dB gain compression point to allow the amplifier to operate as efficiently as possible with minimum distortion. Operating conditions of the power amplifier such as extreme temperature variations affect characteristics of the power amplifier, for example, the power amplifiers 1 dB gain compression point, causing the power amplifier to operate closer to saturation with extreme temperature conditions.
Turning now to FIG. 1, where an exemplary prior art Power Control Loop (PCL) used in wireless communication systems, such as a GSM system, is illustrated and denoted generally as 10. PCL 10 comprises a Power Amplifier (PA) 12 having a amplifier input and a amplifier output coupled to a RF input 14 for receiving RF input power Pi and a RF output 16 for supplying a linearly amplified RF output power Po. A variable attenuator 18 disposed between RF input 14 and PA 12 comprises a input coupled to RF input 14 and a output coupled to the amplifier input. Variable attenuator 18 further comprises a control input 20 for controlling the attenuation level at RF input 14.
PCL 10 further comprises a directional coupler 22 coupled to RF output 16 for coupling a portion, E0, of output power Po through the feedback path. PCL 10 further comprises a RF power detector 24 for rectifying and linearising coupled power E0. RF power detector 24 comprises a detector input coupled to directional coupler 22 and a detector output containing a detected signal VD. RF power detector 24 may comprise a detector circuit for rectifying the coupled power Eo and a linearizer for logarithmically amplifying detected voltage VD. 
PCL 10 further comprises a comparator 30 having an inverting input 32 coupled to detector output containing detected voltage VD and a non-inverting input coupled to a supply reference terminal 34 containing a supplied reference voltage VR. Supplied reference voltage VR supplied by an external source such as a Digital Signal Processor or Applications Specific Integrated Circuit (ASIC) is a correct representation output power Po. Comparator 30 compares detected signal VD and reference signal VR and supplies the difference, an error signal VE, which is filtered through a loop filter 36. The filtered signal, a control signal VC, adjusts the attenuation level correcting any deviation in output power Po.
Refering now to FIG. 2, where a schematic view of prior art RF power detector 24 is illustrated. RF power detector 24 comprises a detecting diode 50 for rectifying proportional power Eo. Detecting diode 50 is coupled to a detector input 52 through a coupling capacitor 54 and a input matching circuit 56. Detecting diode 50 is coupled to a detector output 58 through a lineariser 60. RF power detector 24 further comprises detecting diode 50 coupled to detector output 58 through a lineariser 60. RF power detector 24 further comprises a RC circuit 64 comprising a capacitor 66 coupled in series with a resistor 68. RC circuit 64 is coupled in parallel between lineariser 60 and detecting diode 50. RC circuit may be tuned to control the amount of detected voltage Vd seen across RC circuit 64.
PCL 10 uses negative feedback to control the gain of PA 12 so that PA 12 ramps up, transmits and ramps down within the specified time and with minimum distortion. Error signal VE is used to adjust the operating point so that output power Po is maintained within requirements. However, if PA 12 is operating at extreme temperatures, characteristics of PA 12 vary affecting the efficiency and linearity of the device. For example, the 1 dB gain compression decreases causing the margin between the operating point and the 1 dB gain compression point to decrease. Decreasing the margin between the operating point and the 1 dB gain compression point causes the amplifier to operate too close to saturation resulting in distortion and non-linearity which produces spectral growth.
As may be seen an improved apparatus to adjust the operating point of a RF power amplifier according to extreme temperatures could prevent RF power amplifier saturation under extreme temperature conditions.
The present invention presents an improved apparatus for preventing power amplifier saturation when used in power control circuits.
The invention provides a compensated RF power detector utilized in power control circuits for preventing power amplifier saturation. A power amplifier uses a power control circuit to maintain the power amplifier as efficiently and linearly as possible without distortion. The operating point of the power amplifier is defined as close to the 1 dB gain compression point as possible for maximum efficiency and linearity with minimum distortion. However, extreme temperature conditions affect the 1 dB gain compression point causing the power amplifier to operate too close to saturation resulting in distortion or spectral growth.
In an embodiment, the compensated RF power detector comprises a RF power detector and a variable RC circuit. The RF power detector has an RF input for receiving RF power, a detector output for supplying a compensated detected voltage and, a detecting diode for generating a detected voltage in response to the received RF power. The detecting diode has an anode coupled to the RF input and a cathode coupled to the detector output. The variable RC circuit comprises a capacitor and a control circuit. The capacitor has a first lead coupled to the cathode of the detecting diode and a second lead coupled to the control circuit. The control circuit has a control output containing a varying resistance coupled to the second lead of the capacitor. The variations in the varying resistance result in the compensation of the detected voltage.
In the embodiment, the RF power detector further comprises a coupling capacitor having a first lead coupled to the RF input and a second lead coupled to the anode of the detecting diode. The RF power detector further comprises an input matching circuit disposed between the coupling capacitor and the detecting diode and, a lineariser disposed between the PIN diode and the RF output.
In the embodiment, the control circuit further comprises a PIN diode having an anode and a cathode, with the cathode of the PIN diode coupled to a ground potential and the anode of the PIN diode coupled to the control output. The PIN diode generates the variable resistance at the control output. The control circuit further comprises a bias voltage source coupled to the anode of the PIN diode for supplying a bias current. A temperature dependent resistor is disposed between the PIN diode and the bias voltage source and has a first lead coupled to the bias voltage source and a second lead coupled to the anode of the PIN diode. The resistance of temperature dependent resistor varies across different temperatures and affects the amount of bias current supplied to the PIN diode according to temperature.
In the embodiment, the control circuit comprises a resistor disposed between the temperature dependent resistor and the PIN diode. The control circuit further comprises a capacitor having a first terminal coupled to ground potential and a second terminal coupled between the resistor and the temperature dependent resistor. The control circuit further comprises a inductor disposed between the resistor and the anode of the PIN diode.
In an alternative embodiment, the control circuit comprises a PIN diode having a cathode coupled to ground and a anode coupled to the control output. The PIN diode generates the variable resistance at the control output. The control circuit further comprises a bias voltage source for biasing the diode and an operational amplifier having a output containing a temperature dependent voltage coupled to the anode of the PIN diode through a second resistor. The operational amplifier has a non-inverting input coupled to the bias voltage source through a temperature dependent voltage divider and a inverting input coupled to the temperature dependent output through a third resistor and to ground potential through a fourth resistor.
In the alternate embodiment, the temperature dependent voltage divider further comprises a temperature dependent resistor and a second resistor. The temperature dependent resistor having a first lead coupled to the bias voltage source and a second lead coupled to the inverting input. The second resistor coupled from ground potential to the second lead of temperature dependent resistor.